Drawing from a consideration of the cellular basis of EEG biofeedback and an understanding of neuronal functions, it is possible to view EEG and EEG operant training as a form of normalization training that emphasizes brain self-regulation, in an objective and scientifically driven approach towards integrated brain function.
It is instructive to begin at a basic level, which is that of individual cortical brain cells. FIG. 1 is an interior view of a typical neuronal assembly (not to scale), and shows a figurative view of layers I-VI of a cortical neuronal assembly. The cells marked “P” are the pyramidal cells, which are the primary processing elements in the neocortex. This view is the same in various areas of the cortex, and is thus applicable whether the cortex is sensory, perceptual, executing motor control, planning, or memory. In all cases, the pyramidal cells are mediated by an extensive network of interneurons (marked “H”, “F”, “B”, “N”, “M”, and “S”) which communicate between and among themselves, as well as with the pyramidal cells. The majority of interneurons are inhibitory. Without this inhibitory influence, incoming upstream (“afferent”) neuronal impulses would produce an overabundance of action potentials in the downstream (“efferent) neurons, ultimately leading to a chaotic excess of meaningless activity.
The inhibitory interneurons have significant influence, and condition the downstream neurons so that action potentials can only be produced as a result of persistent, accumulated afferent signals. By modulating the extent and magnitude of the inhibitory interneuronal activity, the brain can tone down activity, so that the cortex generally has a manageable level of activity, providing useful information processing and control. Another manifestation of essential inhibition is “lateral inhibition” in which adjacent neurons have a tendency to inhibit each other's activity. This phenomenon is essential to retaining the acuity of sensory processes, as it prevents the spreading of incoming activity, and ensures that a fine level of detail can be preserved as signals are conducted from the peripheral sensory organs, through sensory pathways, into and through the sensory areas of the cortex.
The EEG sees the millivolt-level postsynaptic activity of pyramidal cells in the form of microvolt-level surface potentials that are conducted from the cortex to the scalp via volume conduction. It is when pyramidal cells polarize synchronously that they produce a measurable potential. Generally, the action of these cells is not highly synchronized, so that their external potentials cancel out at the scalp. However, when even a small number of cells polarize in a synchronous fashion, they produce a measurable surface potential. This phenomenon is so extreme that less than 5% of the pyramidal cells are capable, when synchronized, to control over 95% of the overall EEG. EEG signals are further spread or “smeared” as they reach the cortex, so that a given surface sensor is able to detect activity not only from the cortex directly below, but also from areas distant from the sensor, as shown in FIG. 2.
FIG. 2 shows the effect of EEG “blurring” on the distribution of surface potential produced by a given cortical generator, where the signal is maximal near Fz, yet is detectable in varying amounts across the entire scalp. As a rule of thumb, approximately 50% of the energy detected by a 10-20 site is produced by the cortex lying below the sensor, while the other 50% is produced by adjacent, as well as more distant, sites.
FIG. 3 schematically shows a figurative view of the combined activity of multiple neuronal assemblies, their interconnections, and the production of BEG. This simplified representation shows that there are multiple neuronal assemblies, all interconnected in various ways, and all producing their contribution to the overall BEG signal. Whereas the synchronous activity of a given neuronal assembly can produce its portion of the EEG, the connections among neuronal assemblies are responsible for the connectivity measurements (coherence, phase, etc) that can be measured between sites.